This invention relates to vehicle brake by wire systems, and more particularly to an actuator for use in a vehicle brake by wire system.
Vehicle braking systems, especially automotive braking systems, have typically been hydraulic-based. Hydraulic systems convert fluid pressure into linear and/or mechanical motion. Such systems allow the source of the hydraulic pressure to be positioned remotely from the cylinders that effect the braking action. These systems normally include an actuator, such as a brake pedal, reservoir fluid (such as in a master cylinder) which is responsive to pressure applied by the actuator, and means such as fluid cylinders for converting the hydraulic pressure to a braking force. Mechanical braking pressure is achieved by utilizing the force of the depression of the brake pedal by the driver to increase the pressure on the master cylinder. Such systems are typically accompanied by a vacuum boost that multiplies the force supplied to the brake pedal, throughout the braking operation. The increased pressure in the master cylinder is then transmitted through fluid lines to the fluid cylinders. The fluid cylinders operate calipers, thereby forcing the calipers and brake pads against the rotors and/or drums which slows the vehicle by frictional force.
Hydraulic systems of this type have several disadvantages. The master cylinder, vacuum booster, ABS modulator and hydraulic lines all take up space and add weight to the completed vehicle. Installation of standard hydraulic braking systems is also complicated and labor intensive. Additionally, the large number of parts and installation also adds to repair and maintenance issues as individual parts reach the end of their useful life.
A variant form of applying a vehicle brake system is referred to as a brake by wire (BBW) system. BBW describes the ability to activate vehicle wheel brakes via an electric signal generated by an onboard processor/controller as a result of input signals thereto. Brake torque is applied to the wheels without direct mechanical interaction between the vehicle""s brake pedal and the wheel brake.
A particular type of BBW systems is known as a xe2x80x9cdry interface cornerxe2x80x9d (DIC) system. The DIC system is known as a hybrid system, in that electric signals are used to generate the type and amount of braking force required at each wheel of the vehicle with electrical wires rather than standard hydraulic brake lines. The typical DIC system operates when a driver inputs a force to the brake pedal. A force sensor and travel sensor attached to the pedal transmits an electronic signal to an electronic controller, which in turn sends the signal to a self-contained braking module typically located at each wheel of the vehicle. The self-contained braking module takes the electrical signal and mechanically brakes the vehicle. The self-contained module utilizes an individual motor that drives a ball screw piston assembly, which in turn pressurizes hydraulic brake fluid to ultimately apply the brake caliper to a rotor at that corner of the vehicle. Such a DIC system significantly reduces assembly cost. The individual modules can be separately assembled and fluid filled prior to the manufacture of the vehicle. DIC modules then only need to be bolted to the automobile during the assembly process and plugged in using standard electrical connections. Thus, reliability and quality control of the overall brake system is also increased. Finally, the elimination of hydraulic lines stretching throughout the vehicle as well as the elimination of the master cylinder, booster, and ABS modulator reduces space requirements within the engine compartment.
A DIC brake actuator normally has a fluid reservoir that is used to compensate for long term brake lining wear. Acuators of this type are described in patent application Ser. No. 09/769,644 and patent application Ser. No. 09/792,727, now U.S. Pat. No. 6,298,961, both of which are assigned to the assignee of the present invention and hereby incorporated by reference. When the actuator in these systems in at a released position, the reservoir communicates to a bore of the actuator through a bypass hole. When the actuator piston is applied, no fluid is displaced to the wheel brake until the bypass hole is covered by a seal of the actuator piston.
The present invention is an actuator assembly for a vehicle brake system. The actuator assembly comprises a piston disposed in a bore, a reservoir, and a solenoid. A channel is in fluid communication between the bore and the reservoir, and the solenoid is disposed in the channel. The solenoid is movable between an open position and a closed position. In the closed position, the solenoid blocks fluid communication between the bore and the reservoir.
Because the time required to activate the solenoid is less than the time needed to initiate braking, there is no displacement loss at the beginning of a brake event, and response time is improved. The present invention also improves anti-lock braking system control on low coefficient surfaces. Additionally, the present invention simplifies the overall hardware configuration of a BBW system, and in particular eliminates the need in systems with bypass holes to closely control part tolerances in order to ensure proper closure of the bypass holes.
These and other features and advantages of the invention will become further apparent from the following detailed description of the presently preferred embodiments, read in conjunction with the accompanying drawings. The detailed description and drawings are merely illustrative of the invention rather than limiting, the scope of the invention being defined by the appended claims and equivalents thereof.